Based on studies showing that a 472-U - 472-C mutation in the Sabin poliovirus strain type 3 is a major determinant of reversion to neurovirulence, we have developed a novel molecular assay for the revertant particles in preparations of the vaccine. It was found that the proportion of 472-C revertants was significantly higher in those batches of the vaccine that had failed monkey neurovirulence test. During even a few passages of the virus in African green monkey kidney cells the revertant frequency was rapidly increasing. The results suggest a potential alternative to the monkey test. A patent application for the method has been submitted and a manuscript for publication prepared.